Telomerase is the enzyme that maintains chromosome ends (telomeres), by synthesizing repetitive telomeric DNA. This prevents telomere erosion during chromosome replication, and thereby prevents the genetic instability and cellular senescence that occurs when telomeres become critically short. However, a compelling set of observations now support the hypothesis that telomerase has a second, telomere- independent function which also promotes cell growth and division, inhibits cell death, and contributes to oncogenic cell transformation. Our recent studies provided the first mechanistic insight into this new function for telomerase, by showing that in human mammary epithelial cells telomerase modulates the expression of a set of twelve genes that may directly promote cell proliferation and inhibit cell death. The specific work of this proposal will be to understand biological effects of the program of gene expression induced by telomerase. First, we will study the effects of telomerase-regulated genes, individually and in combination, on mammary cell proliferation and cell death, and using a three dimensional in vitro model of epithelial acinar development we will study their effects on mammary morphogenesis. Second, we will identify the domains in telomerase that mediate its ability to induce and repress gene expression. Based on these studies we will construct separation-of-function mutations, which will be used to determine the specific contributions that telomere maintenance and regulation of gene expression make to cell survival, senescence and proliferation. Third, we will use telomerase mutants and telomerase-regulated genes to test the hypothesis that modulation of gene expression by telomerase is essential for its ability to promote cell transformation. These experiments will use two- and three-dimensional cell culture models, and transgenic mouse models of oncogenic transformation. Collectively, these studies will have important impact on our understanding of the mechanisms by which telomerase promotes tumorigenesis independently of telomere maintenance. This knowledge would directly impact our thinking about telomerase's role in cancer and will be critical for the development of effective therapies targeting the oncogenic functions of telomerase and/or its downstream signaling pathways.